Polyolefins, particularly polyethylene, may be made with a broad range of properties. Especially preferred are high density polyethylene ("HDPE") and linear low density polyethylene ("LLDPE") made at low reaction pressures, typically 100 to 300 psi, and reaction temperatures of less than 100.degree. C. HDPE and LLDPE can be used in many applications, such as films, injection molding, extrusion coating, rotational coating, blow molding, pipe, tubing, and wire and cable coatings. The properties of articles made from polyethylene often depend on the properties of the polyethylene, such as its molecular weight distribution, comonomer content, and so forth. These properties are in turn influenced by both the reaction conditions and the type of catalyst used to make the polyethylene.
Catalysts based on a variety of transition metals, such as titanium, vanadium, chromium and zirconium are known to be useful for producing polyethylene, and each type of catalyst produces polyethylene having different properties. For example, titanium-based catalysts tend to produce polyethylene with a narrower molecular weight distribution, lower chain termination response to hydrogen and lower comonomer incorporation efficiency, while vanadium-based catalysts tend to produce polyethylene with a broader molecular weight distribution, higher hydrogen chain termination response and a higher level of comonomer incorporation. Moreover, because titanium-based and vanadium-based catalysts each require unique reaction conditions to optimize their productivities, conducting polymerization with either a titanium-based or a vanadium-based catalyst in a single reactor normally dictates that reaction conditions be set within a relatively narrow range. The properties of the polyethylene produced using a single metal catalyst in a single reactor are therefore usually not subject to great variation.
One technique used to expand the operability range of processes for producing broad molecular weight polyethylene is to use a single metal catalyst in two or more staged reactors connected in series. For example, U.S. Pat. Nos. 5,047,468; 5,126,398; and 5,149,738 each relate to the preparation of polyethylene using a titanium-based catalyst in at least two fluidized bed reactors connected in series. Each reactor is operated at certain polymerization conditions such that ethylene copolymer having a high melt index is formed in one reactor and ethylene copolymer having a low melt index is formed in another reactor. The contents of one reactor are transferred to the immediately succeeding reactor, so that the final product is an intimate blend of the high and low melt index copolymers. Although each reactor may be operated under somewhat different reaction conditions, the conditions in each reactor are still limited by the operability range of the titanium-based catalyst.
Mixed metal catalysts in a single reactor have also been used to produce polyolefins having a broader range of properties. For example, U.S. Pat. No. 4,154,701 and DE 3 324 136 A1 relate to olefin polymerization using titanium/vanadium/zirconium catalysts. U.S. Pat. No. 3,899,477 and EP 0 131 420 A2 relate to olefin polymerization using titanium/vanadium catalysts and organoaluminum cocatalysts. This technique, however, is limited insofar as each metal in the catalyst must be active simultaneously in one reactor. This severely limits the operating range of the process, and therefore the range of polyethylene products that can be produced.
U.S. Pat. No. 4,918,038 describes a process for the production of ethylene homopolymers and copolymers having broad and/or bimodal molecular weight distributions using a catalyst system comprising vanadium-containing compounds and a zirconium-containing complex. Although the patent states that this process may be conducted in a single reactor or in a series of reactors, a single reactor is preferred due to efficiency and cost.
It has been discovered that the use of two staged reactors, one containing a titanium-based catalyst and the other containing a vanadium-based catalyst, or preferably both reactors containing a mixed metal titanium/vanadium catalyst, advantageously produces polyethylene having a broad molecular weight distribution. Polyethylene produced by this process is a mixture of titanium-derived polyethylene and vanadium-derived polyethylene. In particular, such polyethylene advantageously has a broad molecular weight distribution that is a combination of the molecular weight distributions of titanium-derived polyethylene and vanadium-derived polyethylene.
According to the invention, a halohydrocarbon promoter is present in one but not both of the reactors, where it activates vanadium catalytic sites and poisons titanium catalytic sites. In the reactor in which the halohydrocarbon promoter is not present, vanadium catalytic sites are inactive and titanium catalytic sites are active.
Specifically, in the case of one reactor containing a titanium-based catalyst and the other reactor containing a vanadium-based catalyst, the halohydrocarbon promoter is present in the reactor in which the vanadium-based catalyst is present. The halohydrocarbon promoter is substantially absent from the reactor containing the titanium-based catalyst.
In the case of both reactors containing a mixed metal titanium/vanadium catalyst, the halohydrocarbon promoter is present in the reactor in which it is desired to activate only the vanadium catalytic sites. The halohydrocarbon promoter is substantially absent from the reactor in which it is desired to activate the titanium catalytic sites.